Resonant pickoff device



2 Sheets-Sheet 1 J. T. BUCKINGHAM ET AL RESONAN'I PICKOFF DEVICE May 23, y1967 'original Filed April 2'7, 1960 |.P|| nlm-,HMH wwVMMM-HIMH n mam 525W WS dww w i nu. I .f ma M ,if E y SL" E Ef, JH W E" iz: Alwin 1i", M, Mur y .f mi QEQ l niv m. :www x. .Y Q "T GLN, `l N @,knou bwcor N\ Il a United States Patent Office 3,321,753 Patented Mayr 23, 1 967 Continuation of abandoned application Ser. No. 25,115, Apr. 27, 1960. This application Feb. 26, 1965, Ser.

3 13 claims. (ci. 340-196) This application is a continuation of application Ser. No. 25,115, filed Apr. 27,1960, now abandoned.

This invention relates to a resonant pickotf device and, more particularly, to resonant pickoif devices particularly adapted for use in accelerorneters.

In copending application Ser. No. 794,487, filed Feb. 4, 1959, now United States Letters Patent No. 3,074,279, issued J an. 22, 1963, there is disclosed a pickoff device which includes two coils. A vane or paddle is used for changing the coupling between` the two coils and for loading the larger lof the two coils. Although pickoff devices constructed in this manner have operated very satisfactorily, the gain obtaina-ble from such devices has been limited as lhas been the maximum paddle spacing. There is, therefore, a need for `a pickoff device which has increased sensitivity and -whichamay be used with greater paddle spacing.

In general, it is an object of the present invention to provide a pickolf device which has substantially increased gain over pickoif devices heretofore provided.

Another object of the invention is to provide a pickoff device of the yabove character which may be utilized with relatively great paddle spacing.

Another object of the invention. -is to provide a pickoff device of the above character Vin which a tuned circuit is utilized.

Another object of the invention is to provide a pickoif device of the above character which is tuned for resonance at a predetermined paddle spacing.

Another object of the invention is to provide a pickoff device of the above character which is detuned rapidly for paddle spacing less than or greater than the predetermined paddle spacing. Y

Another object of the invention is to'provide a pickoif device of the above character which can operate on either side of the resonance curve and which will rundown one of the sides of the resonance curve as it is detuned.

Another object of the invention is to provide a pickoff device of the above character in which the gain is increased as the frequency is increased.

Another object of the invention isto provide a pickoif device of the above character which is particularly useful in balanced systems, wherein the drive voltage is controlled to control the total signal flowing in the output stage, j-by utilizing the difference of two signals obtained by moving one paddle closer and the other farther away from two balance coils.

Another object of the invention is to provide a pickoif device of the above character which is particularly useful with a stable-zero electronic circuitry.

Additional objects and features of the invention will appear from thefollowing description in' which the preferred embodiments have been set forth in detail in conjunction with the following drawings.

Referring to the drawings: VFIGURE 1 is Ia block diagram of a resonant pickoff incorporating our invention.

FIGURE 2 is a block diagram of a modification of FIGURE l incorporating our invention.

FIGURE 3 shows gain curves for different drive voltages when-the tuned circuit is tuned for resonance at a kpaddle spacing of 25 milli-inches.

yFIGURE 4 is an isometric View of the positioning of the windings and the paddle of the pickoff shown in FIG- URE 1. v

FIGURE 5 is an isometric view of the positioning of the windings and the paddle of the pickoff shown in FIG- URE 1.

In general, the resonant pickoff device consists of a transformer which has at least two windings. Means is provided for supplying alternating current of a predetermined frequency to one of the windings. `A paddle of conducting material is movable towards and away from the transformer and means is connected to another of the windings of the transformer to provide a tuned circuit at the predetermined frequency and at a predetermined spacing of the paddle from the transformer. Means is also provided for detecting a change in the output of one of the windings.

In FIGURES l and 2 are shown two embodiments of our invention. In FIGURE l, the resonant pickoff device consists of a transformer lllwhich has three windings L1, L2 and L3. The windings may be wound in any desired manner; however, it is preferable that the three windings be in the form of relatively flat pancake-type c oils as shown .in FIGURE 5. WindingsL2 and L3 are normally coaxial, whereas Winding L1 is spaced from the windings L2 and L3 to provide relatively loose coupling between the winding L1 and the windings L2 and L3 for a purpose hereinafter described.

A signal generator 12 having a desired frequency is connected to the transformer L1 as shown in thedrawing. L2 may be termed a floating winding and means is provided in the form of a capacitor C1 for connecting the winding L2 into a tuned circuit consisting of the winding L2 and the capacitor C1.

The separate winding L3 has been provided so that the output from the transformer has the desired impedance as, for example, an impedance which can be utilized for driving the amplifier 13. A paddle or vane of suitable conducting material such as aluminum or iron is disposed in the vicinity of the transformer 11 and is movable towards and away from the transformer, which movement as hereinafter described causes the amplifier 13 to apply a signal to the detector 16 which has an output which is a function of the paddle spacing.

Operation of the circuit as shown in FIGURE 1 may now be briefly described as follows. Let it be assumed that the tuned circuit consisting of the Capacitor C1 and the windingLZ is tuned for resonance at the frequency of they source 12 and at `a predetermined separation or spacing between the paddle 14 and the transformer 11. As the paddle 14 moves closer to the transformer, it moves closer to al1 of the windings L1, L2 and L3. L1 is normally the farthest from the paddle, whereas windings L2 and L3 are generally at the same spacing from the paddle. As the paddle is moved closer to the transformer 11, the electromagnetic field passing between the coils or windings is disturbed so that the coupling between the coils L1 and L2 and between the coils L2 and L3 is modified or destroyed. At the same time, all the coils are loaded but coils L2 and L3 are loaded more than coil L1 because they are normally closer to the paddle. At the same time the paddle is moving closer to the transformer, the tuned circuit consisting of the capacitor C1 and the inductance L2 is detuned. This detuning of the tuned circuitry together with the modification or destruction of the coupling between the windings and the loading of the windings greatly reduces the signal passing through the coil L3 as the vane or paddle moves closer to the transformer.

The loose coupling between the windings L1 Iand L2 causes these two windings to act like an IF transformer so that a rather abrupt attenuation of all frequencies other than the tuned frequency of the circuit consisting of the capacitor C1 and the inductance L2 occurs.

By way of example, in one embodiment of our invention, the winding L1 consisted of 25 turns of No. 50 wire; winding L2 of 50 turns of No. 50 wire; and winding L3- of 5 turns of No. 50 wire. Windings L2 and L3 were of the coaxial pancake-type with the tightly coupled winding L3 being in the center. Winding L1 was also a pancake-type coil and was spaced from 20 to 50 milliinches from the flat coil consisting of the windings L2 and L3. The signal generator 12 had an output frequency of approximately 2.0 megacycles and the capacitance C1 had a value of approximately 330 micromicrofarads.

In FIGURES 3 and 4 are shown the gain curves which are obtained with such circuitry. In obtaining these gain curves, the frequency of the signal source was varied rather than the size of the capacitance C1 because, as is well known to those skilled in the art, the frequency of the signal source can be more readily varied than the size of a capacitance. The frequency source 12 had a predetermined frequency of 2.0 megacycles when the paddle spacing was 25 milli-inches. Resonance of the tuned circuit was obtained at 2.0 megacycles as indicated by the various curves in FIGURE 3 which are designated by the drive voltages in volts R.M.S. applied by the signal source 12. As will be noted from these curves, maximum output lis obtained at the predetermined paddle spacing of 25 milli-inches. When the paddle is moved closer to the transformer, the gain drops oi very rapidly as is shown by the steepness of the curves. For example, as shown in FIGURE 3, when the paddle spacing is moved from 22.5 milli-inches where the detected current is 160 microamperes, to 2O milli-inches the detected current drops to 100 microamperes. Thus, the gain has been cut f almost in half by 21/2 milli-inches of movement of the paddle. The resonance curve has a peak yand side slopes. The resonance curves have steeper side slopes the closer the paddle spacing approaches zero as can be seen from inspection of the curves in FIGURE 3. Still sharper drop-offs in gain are obtained at other drive voltages as shown by the curves.

In FIGURE 4 are shown the same type of curves with a predetermined paddle spacing of 55 milli-inches. Here, resonance was obtained at a frequency of 1.8 megacycles. Although all portions of the curves are not shown in FIGURES 3 and 4, it is believed that each curve would reach its peak at the same predetermined paddle spacing as shown for the lower curves and drop rapidly in much the same way as the other curves.

The significant thing to be noted from both of these sets of curves is that the gain follows a relatively straight line in relationship with a certain driving voltage up to the point of resonance or the predetermined paddle spacing, after which the gain drops very suddenly and, as a matter of fact, reverses and eventually goes down to zero.

From the foregoing, it can be seen that the output of the resonant pickoi device can be varied greatly by changing the paddle spacing from the predetermined spacing and also by varying the drive voltage.' This is particularly advantageous in that it makes it possible to use the resonant pickoif in a balanced system in which two resonant pickoif devices are utilized. The dilerence between the two pickoff devices is utilized as the paddle moves closer to one pickoif device and farther away from the other pickoff device. This is particularly advantageous for use with our stable-zero electronic circuitry, disclosed in our copending application Ser. No. 25,116,

tiled Apr. 27, 1960, now United States Patent No.`

Although the curves in FIGURE 4 clearly demonstrate that our resonant pickotf device can be used with relatively great paddle spacings such as 55 milli-inches, maximum gain normally can be obtained by tuning the tuned circuit for maximum at the closest possible paddle spacing. This is possible because the other effects of loading and coupling caused by the paddle are strongest near zero spacing. Thus, if absolute maximum gain is essential, this can be obtained by utilizing a closer paddle spacing and then picking the frequency which preferably should be a relatively high frequency. The high frequency is desirable because generally the higher the frequency, the greater the gain. The capacitor and coil for the tuned circuit are chosen for this frequency and tuned for a maximum at this smaller spacing. It must be appreciated that when utilizing such closer paddle spacing the gain will drop off suddenly as the paddle spacing increases beyond the maximum for which the circuit has been tuned. Stops may be provided to prevent movement beyond the optimum paddle spacing.

When it is desirable to operate over a relatively wide range of paddle spacing, then the tuned circuit must be tuned for a maximum at a greater paddle spacing. This, of course, reduces the efects of loading and coupling by the paddle, and, therefore, results in a lower sensitivity to paddle motion.

It should be pointed out that the foregoing description has been given with the assumption that the vane or paddle 14 decreases the inductance of the winding L2 as the vane is moved closer to the winding L2. Such would -be the case if the vane 14 were -made of aluminum. When such a vane is moving closer to the transformer, we would be operating on the lower frequency side of the resonance curve formed by L2 and C1. If, on the other hand, a vane is utilized which increases the inductance of the winding L2 as it is moved towards the winding, such as one of iron or other ferromagnetic material, detuning of the resonant circuit would also occur in the same manner. However, we would be operating on the higher frequency side of the same resonance curve. With such ferromagnetic material, the paddle should be designed so that the loading effects of the paddle rdo not lower the inductance of the winding to thus oppose the effects of the higher permeability material.

With this circuitry, it was found that it was possible to obtain a much greater change in output with the same amount of change in the paddle spacing than was true with circuitry heretofore provided. In fact, it was found possible to obtain a gain improvement of approximately 6:1. This great improvement is caused by the detuning of the resonant circuit which can be described as running down the side of the resonance curve as detuning occurs. The curves, as shown, are substantially V-shaped and drop olf rather rapidly on both sides. In fact, the rate of drop-off depends upon the Q ofthe tuned circuit. The higher the Q of the tuned circuit, the faster will be the rate of drop. Thus, the gain can -be increased by utilizing high-Q inductors and capacitors in the resonant winding.

It is believed that one of the main reasons why a great increase in gain is obtained from our resonant pickoi device is because of the detuning of the tuned circuit consisting of the capacitor C1 and the inductance L2. Detuning is severe because the inductance L2 is rela tively close to the vane or paddle. The winding L1 may form a ipart of the tuned circuit of the driving oscillator. However, since it is only loosely coupled to the windings L2 and L3 and because it is spaced farther away from the vane 14, it is not grossly detuned by movement of the paddle or vane.v Therefore, the results are the same as when an external signal source is used.

In FIGURE 2 is shown another embodiment of our invention which is actuallyy a simplified version of the embodiment shown in FIGURE l. It can be utilized when the impedance of the detector circuit can be properly chosen so that it does not interfere with the operation of the resonant circuit, and for that reason, the additional winding L3 shown in FIGURE l is not required. As shown, the embodiment of FIGURE 2 consists of a transformer 21 having windings L4 and L5. The winding L4 is supplied by a generator 22 having an adjustable predetermined frequency. A vane 23 movable towards and away from the transformer 21 is provided. A tuning capacitor C2 is connected across the winding L5. One side of the winding L5 is grounded as shown, and the other side is connected through a coupling capacitor C3 to a detector 24.

Operation of this embodiment of our invention is very similar to that shown in FIGURE 1. As hereinbefore explained, the additional winding L3 provided in FIG- URE 1 is not required where the imped-ance of the detector 24 can `be matched to the impedance of the tuned winding L5. In the embodiment shown `in FIGURE 2, the winding L5 performs the functions of both windings L2 and L3. L5 and C2 make up the tuned circuit that is tuned in exactly the same manner as L2 and C1 in the circuit of FIGURE l. A signal having an adjustable predetermined frequency is supplied by the generator 22 to the winding L4 and is loosely coupled to the winding L5. The capacitor C1 and the inductance L5 are then tuned for resonance at a predetermined paddle spacing for the paddle 23.

It was found that the circuitry shown in FIGURE 2 operated in much the same manner as the circuitry shown in FIGURE 1 and gave a substantial increase in gain over pickolf devices heretofore provided.

It is apparent from the foregoing that we have provided a new and improved pickoff device which `gives wide variations in output with very small fluctuations in paddle spacin-g, and which is operative with relatively great paddle spacings.

We claim:

1. In a resonant pickoff device, a transformer having at least two windings, said twoV windings 'being loosely coupled with respect to each other, means -for supplying alternating current of a first predetermined frequency to one of said windings, a vane of conducting material disposed outside of said transformer and movable towards and away from said transformer, means connected to another of said windings and forming with said winding a tuned circuit at a second predetermined frequency and at a predetermined spacing of said vane from said transformer, said tuned circuit having a resonance curve in which the gain drops off sharply on yboth sides of the resonance curve as the vane is moved from a position in which it has ra predetermined spacing with respect to said transformer, movement of said vane from the position in which it has a predetermined spacing serving to detune the tuned circuit, to modify the coupling between the windings of the transformer and to modify the load? ing of the windings of the transformer, detector means and means coupling said detector means to the tuned circuit whereby said detector means detects the changes in the tuning of the tuned circuit, the coupling between the windings and the loading of the windings caused vby movement of said vane.

2. A resonant pickoif device as in lclaim 1 wherein said vane of conducting material is formed of a material which decreases the inductance of the winding in the 'tuned circuit -as said vane is moved towards the transvane of conducting material is formed of a material which increases the inductance of the winding in the tuned circuit as said vane is moved ytowards the transformer.

4. A resonant pick-off device las in claim 1 wherein said transformer has first, second and third windings and wherein said detector means includes a detector connected to the third winding for detecting changes in 'the output of said third winding caused by movement of said vane.

5. A resonant pic-koff'de'vice as in claim 1 wherein said two windings are .arranged so that there is a loose coupling between the windings to cause a sharp attenuation of all frequencies other than said predetermined frequency.

`6. A resonant pickoff device as in claim 1 wherein said one winding is spaced farther from said vane than the winding in said tuned circuit.

7. A resonant pickoff device as in claim 1 wherein said means connected to another of said windings forming a tuned circuit includes a capacitor.

8. In a resonant pi-cko device, a transformer having first, second and third windings, the first Vand second windingsfbeing loosely coupled with respect to each other, means for supplying alternating current of a first predetermined frequency to said first winding, a vane of conducting material disposed outside of the transformer and movable towards .an-d away from said transformer, means connected to the second winding forming a tuned circuit at a second predetermined frequency at a predetermined spacing of said vane from said transformer, and detector means connected to said third winding for detecting changes in the signal in the third winding caused by changes in position of said vane to thereby give an indication of the position of the vanel relative to the transformer, movement of said vane causing modificati-on of the coupling between t-he windings to change the signal in said third winding.

9. A resonant pickoff device as in claim 8 wherein said loose coupling `between the first and second windings is obtained by spacing said first winding farther away from said vane than said second winding.

10. A resonant pickoif device as in claim 9 wherein said second and third windings are coaxial and are in the form of relatively flat pancake-type coils and wherein the vane is at least of substantially the same size as said coils.

11. In a resonant pickol device, a transformer having first and second windings loosely coupled with respect to each other, means for supplying alternating current of a first predetermined -frequency to one of said windings, a vane `of conducting material posi-tioned outside and away from said transformer and movable towards and away from said transformer, means connected to the other of said windings forming a tuned circuit having resonance at a second predetermined frequency different from said first frequency at a predetermined spacing of said vane from said transformer, said first frequency causing response by said tuned circuit on its side slope, and detector means connected to said other winding for detecting the change in output in said other winding as the spacing of the vane from the transformer is varied to give an indication of the position of said vane, movement of said vane causing modification of the coupling between the windings and of the loading of the windings and detuning of the tuned circuit to thereby cause a change in the output in said other winding.

12. In a resonant pickoff device, a transformer having at least two windings, means for supplying alternating current of a first predetermined frequency to one of said windings, a vane of conducting material disposed outside of and away from the transformer and movable towards andaway from said transformer, means connected to another of said windings forming a tuned circuit at a second 7 predetermined frequency at a predetermined position of said vanewith respect to said transformer, movement of said vane with respect to said transformer causing modification of the coupling between the windings, loading of `the windings and detuning of the tuned circuit, and means for detecting changesrin the output of the transformer resulting from changes in the loading of the windings and the coupling between the windings as the vane is moved.

13. A resonant pickoff device as in claim 12 wherein said transformer has rst, second and third windings and wherein said means for supplying alternating current is connected tothe rst winding, said means connected to another of said windings yforming a tuned circuit is connected to the second winding and said means for detecting changes in the transformer output includes detector means' connected to the third winding.

References Cited bythe Examiner UNITED STATES PATENTS 11/1941 Thompson 340-195 1/196-3 Morris S40-196 

1. IN A RESONANT PICKOFF DEVICE, A TRANSFORMER HAVING AT LEAST TWO WINDINGS, SAID TWO WINDINGS BEING LOOSELY COUPLED WITH RESPECT TO EACH OTHER, MEANS FOR SUPPLYING ALTERNATING CURRENT OF A FIRST PREDETERMINED FREQUENCY TO ONE OF SAID WINDINGS, A VANE OF CONDUCTING MATERIAL DISPOSED OUTSIDE OF SAID TRANSFORMER AND MOVABLE TOWARDS AND AWAY FROM SAID TRANSFORMER, MEANS CONNECTED TO ANOTHER OF SAID WINDINGS AND FORMING WITH SAID WINDING A TUNED CIRCUIT AT A SECOND PREDETERMINED FREQUENCY AND AT A PREDETERMINED SPACING OF SAID VANE FROM SAID TRANSFORMER, SAID TUNED CIRCUIT HAVING A RESONANCE CURVE IN WHICH THE GAIN DROPS OFF SHARPLY ON BOTH SIDES OF THE RESONANCE CURVE AS THE VANE IS MOVED FROM A POSITION IN WHICH IT HAS A PREDETERMINED SPACING WITH RESPECT TO SAID TRANSFORMER, MOVEMENT OF SAID VANE FROM THE POSITION IN WHICH IT HAS A PREDETERMINED SPACING SERVING TO DETUNE THE TUNED CIRCUIT, TO MODIFY THE COUPLING BETWEEN THE WINDINGS OF THE TRANSFORMER AND TO MODIFY THE LOADING OF THE WINDINGS OF THE TRANSFORMER, DETECTOR MEANS AND MEANS COUPLING SAID DETECTOR MEANS TO THE TUNED CIRCUIT WHEREBY SAID DETECTOR MEANS DETECTS THE CHANGES IN THE TUNING OF THE TUNED CIRCUIT, THE COUPLING BETWEEN THE WINDINGS AND THE LOADING OF THE WINDINGS CAUSED BY MOVEMENT OF SAID VANE. 